1. Field of the Invention
The invention concerns an erasable electro-optic storage disk for use in focused laser beam operated data handling systems, and more particularly to a disk consisting of a semiconductor substrate with a storage layer on top of it that is capable of trapping charge carriers and having electric contacts connected to the semiconductor substrate and to the storage layer, respectively.
2. Description of the Related Art
Erasable optical disks are suitable for use in rotary disk data storages often found in data processing systems. They represent a promising alternative to the widely used magnetic disk. Further applications of optical disks are feasible, for instance, in digital optical computing systems where optical devices or arrays having storage capabilities are required to implement logic functions. Here, the optical disk is an alternative to the more commonly used liquid-crystal light valves.
At present, the most widely used storage medium in random access rotating memories is the magnetic disk. However, compact optical disks have become a competitive contender particularly in view of the potentially high data densities.
Size and shape of the bit storage sites are defined by the narrow focal point of a laser beam, making the round bit site only about one micron in diameter.
For erasable optical storage, there are currently two leading contenders: magneto-optic and phase-change technologies. Both techniques employ glass or plastic disks coated with thin films of storage media and use semiconductor laser diodes for recording. However, they differ markedly in the approach to writing and reading information.
Magneto-optic recording relies on heating from a laser and on an externally applied magnetic field to write data onto a disk coated with a thin film of magnetic material. Data are stored magnetically, the direction of magnetization defining the stored bit values.
In phase-change recording, a short burst of laser light converts a tiny spot on the storage media's highly reflective crystalline surface to the less reflective amorphous, or semicrystalline, state. This is done by rapidly heating the material to a temperature above its melting point and then abruptly quenching it, freezing it into the amorphous state.
Critical concerns about these technologies include media stability and reliability, cyclability, i.e., the number of times information can be rewritten, data access and transfer speeds, and the limited availability of lasers with sufficient power.
An alternative technique is provided by the basically different electro-optic storage devices which rely upon optical and electrical properties of the materials and structures used. Again, focused laser beams serve to address and to define the storage sites. Binary data are stored by trapping charge carriers in a continuous storage medium that may, e.g., consist of silicon nitride.
Trapping of charge carriers in silicon nitride has been studied extensively for application in non-volatile memories. The feasibility has, e.g., been demonstrated in an article "The Metal-Nitride-Oxide-Silicon (MNOS) Transistor--Characteristics and Applications" by D. Frohmann-Bentchkowsky in Proc. IEEE, Vol. 8, No. 8, August 1970, p. 1207-1219. Charge can be stored reliably and can be retained for very long periods of time.
Various storage systems employing devices having layers capable of trapping charge carriers have already been described. The following articles published in the IBM Technical Disclosure Bulletin (TDB) are representative of the state of the art:
"Electro-optical Memory with Write, Read and Erase Characteristics" (IBM TDB, Vol. 15, No. 4, September 1972, p. 1237/38). The disclosed structure comprises an electro-optical crystal with two dielectric films having different electron energy bandgaps deposited on one of the crystal surfaces. A light beam generates, in the crystal, a pattern of electrons. Upon application of an electric voltage, the electrons tunnel through the first thin insulator layer and become trapped between the interface state of the two dielectric layers. Since the trapped electrons cannot return, a permanent charge pattern is formed in the crystal. It induces a variation of the index of refraction and information can be read out using polarized light.
"Non-Volatile Semiconductor Electro-Optical Memory Device" (IBM TDB, Vol. 15, No. 11, April 1973, p. 3455/56). Here, the storage structure consists of a semiconductor with a thin silicon dioxide and a silicon nitride layer on top of it. Traps at the oxide-nitride interface become charged, representing stored data, at cell sites defined by a scanning light beam and transparent strip electrodes to which a positive voltage is applied. Data are read by scanning the device with a light beam. At sites with positively charged traps, a change in voltage is induced that can be detected.
"Memory and Solar Cell" (IBM TDB, Vol. 20, No. 8, Jan. 1978, p. 2995). This article describes a single storage cell consisting of a thick SeTe layer with an Al-oxide film and a transparent Al layer on top of it. The oxide layer functions as a capacitor in which a charge, representing data, can be stored when a light spot is focused on the storage site and contacts to the Se-Te and the Al-layer are simultaneously bridged by a low impedance connection. A light beam and a load are applied for reading: by detecting the presence or absence of current flow, which depends on the data stored, the cell conditions can be sensed.
These known systems suffer from deficiencies such as loss of data due to the large leakage or dark currents, charge spreading and electron recombination which are detrimental to reliable storage operations, particularly in high density systems. Also, fabrication processes are critical.